Teixeira - Node Patterns - Databases

LevelDB is a fast key-value store written by Google engineers that has the following characteristics:

• Keys and values are arbitrary byte arrays.
• It stores data sorted by key.
• It provides basic put(key, value), get(key) and delete(key) primitives.
• It allows multiple changes to be performed in one atomic batch.
• It allows you to read a transient consistent snapshot of the data.

LevelDB can be used from within Node.js by installing the level package. When doing so, it compiles and instals LevelDB as a library, which you can then access through JavaScript.

LevelDB is thread-safe, but is not suited to being accessed from multiple processes. This means that you should only have a LevelDB database open from a single Node process. If you have multiple Node processes, a LevelDB database cannot be shared between them.

Also, LevelDB just exposes a simple key-value store API, on top of which a multitude of extension plugins exist, implementing functions as diverse as job queues, map-reduce views, partitioning, replication, locking, network exposure, and others. In this chapter we will explore some of these.

To instal LevelDB you should add the level package to your package.json application manifest using NPM from the command line:

Here we assume you already have a package.json file at the root directory of your application.

This last commands adds the level dependency to your manifest and installs it. Now you can require it and use it inside your application:

db.js:

Here we created a singleton module that exports a LevelDB database. We start by requiring the level module and use it to instatiate the database, providing it with a path. This path is the path of the directory where LevelDB will store its files. This directory should be fully dedicated to LevelDB, and may not exist at the beginning. In our case the path for the database is contained in the DB_PATH envionment variable or, if not defined, defaults to the mydb directory inside the directory of the current source file.

Now, from another module we can get a reference to the database by requiring our singleton db.js module:

app.js:

After this we can use it immediately to put some values:

You can start issuing commands and queries into a LevelDB database object that has recently been created, without having to wait for it to actually open the database. While the database isnt ready for action, the queries and commands are queued.

You can also use the db object to get some values:

or even delete some entries:

Any of these operations take an optional callback as the last argument. When an error happens while performing the operation, this callback gets the error object as the first argument: